Actually, the step motor "back EMF" can be many, many times higher than the supply voltage. Don't believe it? Then try this:
Accelerate a step motor to 3,000 RPM using a 24VDC supply. That should be easy to do if the motor is unloaded.
Now, disconnect the motor, put an AC voltmeter across one winding and spin the stepper with a drill-press or other motor to 3,000 RPM.
I just did that using a servo motor to spin the stepper and I measured 127V across the stepper at 3,000 RPM. The "back EMF" was 5.3 times higher than the supply voltage!
How is this possible? It has to do with the driving phase angle. At high speeds, the motor phase lags the drive's phase by the ratio formed by the supply voltage and the motor's "back EMF" voltage.
When you rapidly decel the motor from high speed (or desynchronize it), the motor phase advances and this "hidden" voltage appears on the drive's power supply pins. This phenomena is called "load dump" or "returned energy".
Mariss |